The long-term objective of this study is to define the molecular mechanisms by which Dok-1 inhibits p210Bcr-Abl-induced signaling events that lead to neoplastic transformation. p210Bcr-Abl, a constitutively active tyrosine kinase, is the causative agent of chronic myelogenous leukemia (CML), a disorder of hematopoietic cells that occurs with a frequency of about 1 in 100,000 people a year. It is well established that the tyrosine kinase activity is crucial to the transforming capacity of p210Bcr-Abl. Indeed, inhibition of the p210Bcr-Abl kinase with specific Abl kinase inhibitors, such as imatinib mesylate, leads to disease remission - making treatment of CML a successful paradigm of targeted cancer therapy. However, most patients receiving these drugs retain residual leukemic cells, and some develop drug resistance due to the acquisition of mutations in the Abl kinase domain, which eventually leads to relapse of CML. Therefore, new therapeutic strategies need to be developed to treat CML cases resistant to Abl kinase inhibitors. In this context, it is of utmost importance to elucidate the critical signaling pathways and components that moderate p210Bcr-Abl-driven leukemogenesis. We have found that Dok-1, an adaptor protein constitutively tyrosine phosphorylated by p210Bcr-Abl in chronic phase progenitor cells of CML patients, inhibits mitogenic signaling and opposes oncogenic transformation triggered by p210Bcr- Abl. Inactivation of Dok-1 in mice accelerates the onset of the CML-like disease induced by p210Bcr-Abl, whereas expression of Dok-1 diminishes the proliferative activity of p210Bcr-Abl expressing cells. These findings provide a unique framework for the acquisition of novel insights into the molecular underpinnings of p210Bcr-Abl- mediated oncogenic signaling and mechanisms that counteract it. This application aims to delineate the molecular mechanism(s) by which Dok-1 interferes with p210Bcr-Abl-mediated signaling and oncogenesis. Towards these goals, the first specific aim will define the determinants in Dok-1 that are critical for its inhibitory effect on p210Bcr-Abl-driven transformation and leukemogenesis. Specific aim 2 will delineate and characterize signaling pathways Dok-1 acts upon to inhibit p210Bcr-Abl-induced oncogenic transformation. Molecular, biochemical, and cell biological approaches, as well as mouse models, will be used to achieve these objectives. The third specific aim will identify Dok-1 interacting proteins important for its function in p210Bcr-Abl- mediated oncogenic signaling, by using biochemical purification techniques combined with Multidimensional Protein Identification Technology. Information gained from these studies will not only provide novel insights into Dok-1's function and mode of action, but will also contribute to a better understanding of the critical downstream events in the p210Bcr-Abl signaling cascades that drive aberrant expansion of CML progenitor cells. Understanding how Dok-1 inhibits p210Bcr-Abl oncogenic signaling could lead to the conception of improved strategies to control CML and possibly other neoplastic diseases as well.